Typically, variable cam timing (VCT) devices are hydraulic in nature and use pressurized engine oil to change the phase between the camshaft and the crankshaft using a phaser having a rotor and a stator where the stator has pressure chambers and the rotor has vanes in the pressure chambers. Oil is pumped into the chambers to rotate the rotor vanes and cause a shift in phase.
Such hydraulic devices require an accurate control of the oil leakages. Such a requirement can be difficult and the difficulty is enhanced due to the engine conditions and operating temperatures.
Devices for mechanically controlling the phase shift are known, see, for example, U.S. Pat. Nos. 5,012,774 and 5,638,782.